[PATCH v3] x86/vdso: Handle clock_gettime(CLOCK_TAI) in vDSO

[Matt Rickard]

Process clock_gettime(CLOCK_TAI) in vDSO.
This makes the call about as fast as CLOCK_REALTIME and CLOCK_MONOTONIC:

  nanoseconds
 before after clockname
   ---- ----- ---------
    233    87 CLOCK_TAI
     96    93 CLOCK_REALTIME
     88    87 CLOCK_MONOTONIC

Signed-off-by: Matt Rickard <matt@xxxxxxxxxxxxxxx>
---
 arch/x86/entry/vdso/vclock_gettime.c    | 25 +++++++++++++++++++++++++
 arch/x86/entry/vsyscall/vsyscall_gtod.c |  2 ++
 arch/x86/include/asm/vgtod.h            |  1 +
 3 files changed, 28 insertions(+)

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index f19856d95c60..91ed1bb2a3bb 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -246,6 +246,27 @@ notrace static int __always_inline do_monotonic(struct timespec *ts)
 	return mode;
 }
 
+notrace static int __always_inline do_tai(struct timespec *ts)
+{
+	unsigned long seq;
+	u64 ns;
+	int mode;
+
+	do {
+		seq = gtod_read_begin(gtod);
+		mode = gtod->vclock_mode;
+		ts->tv_sec = gtod->tai_time_sec;
+		ns = gtod->wall_time_snsec;
+		ns += vgetsns(&mode);
+		ns >>= gtod->shift;
+	} while (unlikely(gtod_read_retry(gtod, seq)));
+
+	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
+	ts->tv_nsec = ns;
+
+	return mode;
+}
+
 notrace static void do_realtime_coarse(struct timespec *ts)
 {
 	unsigned long seq;
@@ -277,6 +298,10 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
 		if (do_monotonic(ts) == VCLOCK_NONE)
 			goto fallback;
 		break;
+	case CLOCK_TAI:
+		if (do_tai(ts) == VCLOCK_NONE)
+			goto fallback;
+		break;
 	case CLOCK_REALTIME_COARSE:
 		do_realtime_coarse(ts);
 		break;
diff --git a/arch/x86/entry/vsyscall/vsyscall_gtod.c b/arch/x86/entry/vsyscall/vsyscall_gtod.c
index e1216dd95c04..d61392fe17f6 100644
--- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
+++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
@@ -53,6 +53,8 @@ void update_vsyscall(struct timekeeper *tk)
 	vdata->monotonic_time_snsec	= tk->tkr_mono.xtime_nsec
 					+ ((u64)tk->wall_to_monotonic.tv_nsec
 						<< tk->tkr_mono.shift);
+	vdata->tai_time_sec	        = tk->xtime_sec
+					+ tk->tai_offset;
 	while (vdata->monotonic_time_snsec >=
 					(((u64)NSEC_PER_SEC) << tk->tkr_mono.shift)) {
 		vdata->monotonic_time_snsec -=
diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index fb856c9f0449..adc9f7b20b9c 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -32,6 +32,7 @@ struct vsyscall_gtod_data {
 	gtod_long_t	wall_time_coarse_nsec;
 	gtod_long_t	monotonic_time_coarse_sec;
 	gtod_long_t	monotonic_time_coarse_nsec;
+	gtod_long_t	tai_time_sec;
 
 	int		tz_minuteswest;
 	int		tz_dsttime;